"death do us part" or "do us part"

: And, aside from its origin and meaning, could anyone please help me analyze its grammer structure? the "us" and "part" here are very protruding. What does "sth. do sth. part" mean?

Interesting phrase, therefore an interesting question. First, a couple of comments about the form of the qustion. Grammar, not grammer. The latter is a way of saying "grandmother." "Sth." for "something" is chatspeak, which is not spoken here.

"To part" is a verb meaning to separate or divide, as in "part your hair," or "part the Red Sea" or "Let us never part, at least not before graduation." "To do" is also a verb, with a dual meaning. It's an important auxiliary verb, essential in many verb forms. It's also a verb in its own right, meaning to perform an action. Both uses can be seen in, e.g., "I didn't do it," or "Did he do it right?"

In the indicative mood, we conjugate "do" as I do, you do, we do, they do, but he does. So we expect of death, in the indicative, "death does." In the case of "death do us part," the verb is NOT in the indicative, but in the subjunctive, meaning "until death may part us." The "do" here is used as an auxiliary verb.

The quick and dirty precription for the use of the subjunctive is that it is used in conditions contrary to fact. Death has not yet caused us to part. It is also used as a third-person imperative, as in "God shed his grace on thee," meaning "May God shed (distribute, let fall) his grace on thee." This, too, implies a condition contrary to fact, since the phrase implies "in the future."

You've noticed that "us" is oddly located, before instead of after "part," as in "Til death do part us." This, along with the use of the subjunctive, gives the phrase a very old fashioned sound. And why not? That emphasizes the solemnity of the vow and the importance of the occasion.
SS